Geophysical delineation of acidity and salinity in the Central Manitoba gold mine tailings pile, Manitoba, Canada

• Acid mine drainage in the Central Manitoba Mine tailing pile can be mapped geophysically.
• Electrical conductivity–acidity relationship is quantified using analyses of tailing samples.
• Pore-fluid conductivity is due to both sulfuric acid and additional dissolved ions.
• At low pH, Cu2 +, Al3 + and Fe3 + make significant contributions to pore-fluid conductivity.

Surface electrical and electromagnetic geophysical methods can map enhanced electrical conductivity caused by acid mine drainage in mine tailings piles. In this case study, we investigate quantitative relationships between geophysical responses and the electrical conductivity, acidity and salinity of tailing samples at the Central Manitoba Mine tailings in Manitoba, Canada. Previous electromagnetic surveys at the site identified zones of enhanced conductivity that were hypothesized to be caused by acid mine drainage. In the present study, high-resolution EM31 and DC-resistivity measurements were made on a profile through a zone of enhanced conductivity and laboratory measurements of salinity and pH were made on saturation paste extracts from an array of tailing samples collected from the upper 2 m of tailings along the profile.Observed spatial correlation of pH and pore-fluid salinity in the tailings samples confirms that the enhanced conductivity in the Central Manitoba Mine tailings is due to acid mine drainage. Contoured cross-sections of the data indicate that the acid mine drainage is concentrated near the base of the oxidized zone in the thicker parts of the tailings pile. The zone of increased acidity extends to the surface on sloping margins causing an increase in apparent conductivity in shallow penetrating geophysical responses. The quantitative relationship between measured pH and salinity shows that the conductivity increase associated with the acid mine drainage is due only in part to conduction by ions produced from dissociation of sulfuric acid. Comparison of the observations with fluid conductivity estimates based on statistical relationships of pH and ion concentrations in water samples from across the tailings pile shows that Ca2 + and Mg2 + ions also make significant contributions to the conductivity at all values of pH and Cu2 +, Al3 + and Fe3 + ions make additional contributions at low pH. Variability in the measured conductivity at constant values of pH is explained by variations in the contribution of the SO42 − to the measurements. The results of the study demonstrate the feasibility of using electromagnetic geophysical surveys to map acid mine drainage, and related ionic concentration, in mine tailings.